The main stream of axle bearings employed in railroad vehicles is now the sealed type tapered roller bearing by virtue of the compact axle box and maintenance-free characteristic. By the sealed configuration, the temperature in the bearing may rise due to the increase of the inner pressure in the bearing during operation over a long period of time or at high speed. There is also the possibility of adverse effects on the bearing oil seal, depending upon the condition, to cause heat generation of the oil seal itself. For the purpose of preventing such temperature increase in the bearing as well as development of heat at the seal itself, a gas vent plug is provided to avoid increase of the inner pressure in the bearing. The configurations of a gas vent plug and a bearing device attached with a gas vent plug are conventionally disclosed. For example, Japanese Patent Laying-Open No. 6-248908 (Patent Document 1) discloses a general vent structure, and Japanese Patent Laying-Open No. 2003-294047 (Patent Document 2) discloses a bearing with a vent provided at the bearing itself Patent Document 1: Japanese Patent Laying-Open No. 6-248908Patent Document 2: Japanese Patent Laying-Open No. 2003-294047